Conventionally, as an X-ray imaging apparatus, for example, an X-ray computed tomography apparatus (X-ray CT apparatus) has been used. The X-ray CT apparatus is designed to collect measurement data by exposing a subject to an X-ray from an X-ray tube and using an X-ray detector to detect the X-ray that has passed through the subject. The X-ray CT apparatus is designed to rotate the X-ray tube and the X-ray detector, and reconstruct an image on the basis of collected data, which are obtained at each angle, thereby obtaining a CT image.
The X-ray CT apparatus sets the tube voltage and tube current of an X-ray tube, and collects data. However, if the tube voltage and the tube current remain fixed when data is collected by an imaging, the quality of the image varies depending on physical dimensions of the subject (such as body weight and body height). In order to increase the quality of the image, the tube voltage and the tube current are set to higher levels, or either the tube voltage or the tube current is modulated, during an imaging operation.
Moreover, an appropriate X-ray dosage varies according to regions of the subject whose imaging. For example, thick regions of the body, such as a head, thorax, and abdomen are irradiated with a high energy X-ray. Thinner areas Are irradiated with a low energy X-ray. In general, the tube voltage is set to a constant level, and the tube current is modulated, thereby ensuring the quality of an image of each region of the body. The X-ray CT apparatus may employ a dual energy scan. The dual energy scan is a process of imaging operation with the use of two or more types of X-ray energy at varying tube voltages when the images of the same area Are imaging, thereby obtaining an image of each level of energy.
Meanwhile, in the case where the tube current or the tube voltage is varied when imaging, a patient is exposed to a higher dose of radiation if the tube current or the tube voltage is increased, for example. Conventionally, in order to prevent the exposed dose from exceeding a specified value, CTDIvol or DLP is generally displayed on a console as an index of the exposed dose. CTDIVol (CT Dose Index Volume) represents the density of the X-ray, and DLP (Dose Length Product) represents the total dose and CTDIVol and DLP are index values that represent reference doses. The reference doses (CTDIvol, DLP) are set based on the body weight, body height, age, and other factors of a subject, for example.
However, the above displaying process is designed so far only to meet requirements of the IEC standards. There is an example in which the change of exposed dose is numerically displayed. However, in the process of providing numerical information, it is difficult to represent the change of exposed dose of a complex imaging protocol such as tube current modulation (or tube voltage modulation) or dual energy scan.